Margaret MacCampbell, age 14, of Powellton, West Virginia, for her question:

How do the moon's dimensions compare with the earth's?

Most planets are much more massive than their satellites. The earth and the moon are more evenly matched than any other pair in the Solar System. The diameter of the moon is 2160 miles, a little more than a quarter of the earth's diameter. In volume, the earth is 50 times larger than the moon and the earth is 83 times more massive. The differences in volume and mass seem out of line. You might expect the moon to weigh one fiftieth of the earth because it is 50 times smaller. There is a reason why this is not so.

The mass of a heavenly body is the amount of matter packed into its volume. The moon happens to be made of lighter materials. The heaviest materials, of course, are in the central core. The total volume of the earth weighs 5 1/2 times more than an equal volume of water. The total weight of the moon is only about 3 times heavier than an equal amount of water. The surface gravity of the moon depends upon its size and its mass  and it works out to be about one sixth the gravity of the earth.